Outline. Switching protocols 1. SS7 and Frame Relay. Signalling principles. Signalling interfaces. Signalling is hard

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1 Outline Switching protocols 1. SS7 and Frame Relay Signalling principles SS7 & telephone networks X.25 Frame Relay Later weeks: ATM signalling RSVP Internet signalling Another important signalling protocol: SIP 1 2 Signalling principles Signalling: The exchange of information for the purpose of managing a flow of information. In particular, establishing and releasing connections What is signalled: Admission control, billing Routing, e.g. translate 1300 number to select line to call Resource reservation (tentative at first, then confirmed) User is concerned with end-to-end performance. Must translate this to local (per-hop) performance requirements. Often start aggressively to see if the e2e performance is achievable, then relax. Parameter negotiation eliminate options that aren t supported Signalling during a call to change its characteristics, e.g. renegotiate service contract, forward call Signalling interfaces Signalling occurs at two different places: Access signalling: At User- Interface (UNI) e.g. Digital Subscriber Signalling System No. 1 (DSS1 Q.931) signalling: Within the network at NNIs ( Node Interfaces or - Interfaces) e.g. Signalling System No. 7 (SS7) ATM Forum distinguishes between interfaces to Private (P) or Public networks Private interfaces: [ATM Forum] UNI, NNI Public interfaces: UNI, NNI 3 4 In-band vs out-of-band signalling In-band signalling follows same path as payload, in the same channel. UNI: DTMF=in-band, ISDN D channel=out-of-band (payload on separate B channel) Associated signalling: follows same path as payload, but in a separate channel Out-of-band signalling advantages: Higher speed: Signalling channel may offer higher speed (e.g. 56kb/s vs DTMF pulsing) Separating signalling from simpler payload transfer may simplify payload processing Facilitates signalling during the call, rather than signalling only before the call. (assuming circuit-switching) Signalling can follow different path than payload, e.g. via databases not en-route to destination Signalling and payload transfer can evolve separately, e.g. phone signalling advances Easier to ensure conveyance of signalling, despite overload of payload In-band signalling advantages: Detect impairments on path used for payload when signalling e.g. e2e signalling won t succeed when payload can t be transferred. Allows piggybacking of payload with initial signalling, reducing impact of delay Signalling is hard Dumb terminals, e.g. telephones (traditionally), so complex signalling functionality in network Large feature set (extended by competing service providers) + feature interaction Must still be able to extend & maintain Need speed: communication can t progress while signalling Need reliability, e.g. for emergency phone calls Need interoperability between multiple vendors 5 7

2 Telephone signalling UNI: Originally: pulse code for stepping switches Later: Dual-tone multifrequency (DTMF) ( touchtone ): simultaneously send two tones selected from a set of 8 Standard: Q.931/I.451 NNI: Originally: pulse code, same as UNI Later: transmitted in-band, e.g. 1 bit of voice channel used every 2.5ms to indicate loop supervision current Poor security Inflexible: Signalling must follow same path as voice Later: out-of-band signalling. Standard: SS7 Signalling System No. 7 (SS7) Voice trunks originate and terminate at Local Exchanges, and may be switched through Transit Centres. Each Local Exchanges & Transit Centres has a Signalling Service Point (SSP) SSPs exchange signalling information using the SS7 protocol across a Common Channel Interoffice Signaling (CCIS) network Signalling network operates in parallel with information transfer network Note: circuit-switched network is supported with a packet-switched signalling network 8 Figure from TELE 9301 notes for SS7 nodes SS7 network architecture Signal Switching Point (SSP) Originates and terminates SS7 messages Located at end offices, or access tandem devices Runs the Transaction Capabilities Application Part (TCAP) Majority of switching is really done by STPs Drawn as circles Service Control Point (SCP) Provides database operations to another SCP or SSP, e.g. Call Management Services DB: translates 800 numbers into POTS numbers Line Information DB: billing, calling card, collect/reverse-charges services Mobility management: home & visitor location registers Drawn as cylinders SCPs and STPs are installed as mated pairs, each in a different geographic location Signaling Transfer Point (STP) Switches signaling traffic Includes MTP3 and SCCP Drawn as squares with a diagonal line 10 Quad = 2 mated pairs of STPs is hierarchical Each link shown may actually be a link set Figure from TELE 9301 notes for SS7 link types status Letter Name Connects A Access SEP-STP (local) B Bridge same level, but not mates C Cross STPs (mated) D Diagonal different levels E Extended SEP to non-local STP F Fully- SEP-SEP associated 3 basic link states: Out-of-service (OOS) Failed: Can t provide acceptable quality (e.g. BER) Deactivated by management Blocked: Processor (node) outage at one end Inhibited, e.g. standby link Aligning/proving checks that the link works. 2 types: normal alignment (when there exist alternative links) emergency alignment (faster than normal alignment?) Alignment can be lost when receive six consecutive 1s (bit stuffing has failed) or an excessively long Signalling Unit. In-service / busy Status is indicated in Status Signalling Unit (LSSU) A route through a particular STP to a destination may be: Available Restricted: destination is reachable, but alternate route is preferred, e.g. less congested Unavailable SEP = Signalling End Point (SSP or SCP) 12 13

3 Identifying nodes Point codes uniquely identify Signalling Points US: 3 bytes: network-cluster-member, e.g Terminals (e.g. phone lines) are given titles according to the E.164 numbering plan Initial digit of prefix indicates region: 1 North America (same prefix, e.g. US, Canada, Belize) 2 Africa 3 Europe (part) 4 Europe (part) 5 Central and South America 6 Australia and pacific, e.g. +61 for Australia 7 Former USSR 8 China, Japan and Pacific 9 India and Middle East 14 Application Presentation Session Transport Data Physical SS7 protocol stack OMAP: Operations, Maintenance and Administration Part e.g. to diagnose link problems & validate routing tables OMAP ASEs TCAP ISUP SCCP MTP Level 3 MTP Level 2 MTP Level 1 ASEs: Application Service Elements TUP User Parts Services Part Services Part is being replaced with alternative transport, e.g. ATM, SCTP over IP 15 Message Transfer Part 1 Provides the physical layer Signalling links are bidirectional May have a set of links (e.g. 8) connecting adjacent nodes Nominal rate is 56kb/s (US) or 64kb/s (European) 1.5Mb/s ATM links are rare and are considered to be high-speed MTP 2 Provides reliable frame transfer between adjacent nodes Functions: proves the link before use by monitoring error rate Frames SUs, using flags + bit-stuffing to ensure uniqueness of flags and prevent long runs of 1s (loss of alignment ). Detects errors For link quality monitoring For reliable transfer Recovers from errors using go-back-n retransmission. (Also offers a Preventive Cyclic Retransmission scheme for satcom, in which SUs are retransmitted before receiving nack) Flow control: Congested end indicates that it is busy by using a Status Signal & withholding acknowledgements MTP 2 Signalling Unit format Transmission order Flag BSN BIB FSN FIB Len. Spare Service Status CRC Flag n Length in bits optional Flag: For framing SN = Sequence Number IB = Indicator Bit; requests retx n 0 1 or 2 Message type Fill-in Signalling Unit (FISU) Status SU (LSSU) F{SN,IB} = Forward {SN,IB} 8<n 272 Message SU (MSU) B{SN,IB} = Backward {SN,IB} (ack) Len. # of octets between Len & Check (saturates at 63) Aids integrity & indicates type before full receipt. Service field is absent for FISUs/LSSUs Service Information Octet: Indicates type of MSU, domestic/international, and priority Check = checksum Note: SS7 messages are often shown with right to left transmission order. 18 Signalling Units (messages) 3 types: Message SUs: Carry signalling information Status SU: initiate link alignment, indicate processor status, quality of received messages Fill-In SU: Fills link, when nothing else to send. Provides continual error monitoring (for reliability) May also carry acks (BSNs) 19

4 MTP 3 Concatenates links to form end-to-end paths Handles: addressing, routing, alternate routing, congestion control 2 functions: Signalling Message Handling: Message discrimination = packet classification: determine if SU is destined to this signalling point (feed to message distribution to distribute to subsystem within this SP), or another reachable through this signalling point (STP) (feed to message routing component) Signalling Management: Reconfigure in case of failure Control traffic during congestion Messages must not be lost, duplicated or resequenced MTP 2 provides link reliability; MTP 3 provides path reliability (e.g. no loss from STP buffer) Much more complicated than best-effort IP router Signalling traffic management procedures -based operations: Changeover: response to link failure Changeback: response to link recovery Signalling point restart Route-based operations: Rerouting: Forced or controlled. e.g. redirect traffic when a route becomes restricted Signalling route set test Signalling route set congestion test Transfer controlled: response to link congestion: Return control messages to source for all messages received with priority less than threshold Transfer prohibited: When STP has no routes to DPC Transfer restricted: Transfer allowed: Signalling Connection Control Part (SCCP) Partially analogous to TCP operating over IP Provides connection-oriented service & segmentation and reassembly for long messages Allows addressing of subsystems within a node, not just nodes themselves (like TCP port numbers). Examples of subsystems: 800 call processing, callingcard processing, etc. Transaction Capabilities Application Part (TCAP) is the protocol used to communicate with these subsystems, and so runs over SCCP. Partially analogous to network-layer functions: Provides Global Title Translation : A title (e.g number) is translated into a DPC that can be used for routing MTP 3 headers Start with a 7B routing label, indicating Destination Point Code (3B) Originating Point Code (3B) Signalling Selection (SLS) for load balancing. Originator selects at random (keeping same value for SUs that must be kept in sequence) Format of SCCP and ISUP messages ISUP messages include a Circuit Identification Code immediately after the routing label SCCP & ISUP messages contain a message type code followed by Mandatory parameters (depending on type) First fixed-length parameters, then pointers to variablelength parameters Pointer to optional parameters values (length,value) of variable-length parameters Optional parameters (name, length, value) MTP 3 (Routing Label) CIC (ISUP only) Message Type Mandatory fixed param. 1 Mandatory fixed param. m mandatory var. param. 1 mandatory var. param. v optional part mandatory var. param. 1 mandatory var. param. v optional variable 1 optional variable o 25 ISDN User Part (ISUP) To establish & tear-down calls, both voice and data, and ISDN and non-isdn. Replaces Telephone User Part (TUP) Supplementary services: Closed User Groups (Virtual Private s): Calls are permitted only between members of the user group. ISUP messages used to verify membership. Call forwarding Calling Line Identification (& Restriction: don t disclose ID) User-to-user signalling (payload across signalling channel) 26

5 A & B are SSPs that are co-located with voice switches 1. SSP A 1. selects an idle trunk to SSP B 2. sends an Initial Address Message to SSP B Call setup example 2. SSP B receives IAM, determines that target is available 1. Sends ringing tone over trunk 2. Returns Address Complete Message, indicating reverse path trunk Switch A connects caller to trunk, so they hear ringing 3. When target picks up phone, B sends an Answer Message to A, indicating selected trunk 4. Caller hangs up, A generates Release message 5. B responds with Release Complete after disconnecting trunk Figure from A B 28 Transaction Capabilities Application Part Remote Procedure Calls, like ROSE of OSI in particular, to consult databases, e.g.: 1800 numbers 1300 numbers: everyone calls one number (eases advertising), but they are directed to: the local service (e.g. nearest Pizza Hut), using calling number information an available service (e.g. west coast in evening) Local Number Portability: Allows consumers to retain their number when changing carriers, promoting competition between carriers. 30 X.25 Types of Virtual Circuits Designed for character transfer to/from dumb terminals ( low rate), e.g. banking & airline reservations by the CCITT (now ITU-T) dominated by public network operators in mid-1970s when: copper lines were the dominant transmission technology high error rates terminals were dumb, e.g. telephones, screen+keyboard public network concept that the network should provide the service, rather than the terminals network provides strong error protection Mainly important for the legacy that it leaves (e.g. virtual circuits) Amateur (radio) X.25 still used e.g. included in Linux distributions Permanent Virtual Circuits (PVCs): Set up by a network operator, using management tools lead time of days Switched Virtual Circuits (SVCs) dial-up service reduces load on network resources (particularly multiplexing identifiers) Signalling protocol to establish SVCs: Q.933 More complicated than PVCs: Multiple simultaneous requests, more dynamic Access Procedure, Balanced (LAPB) Frame structure: Flag (1B) = 0x7E; LAPB uses bit stuffing to ensure that this flag doesn t occur in the midst of the data, and so indicates the beginning of the frame. Address field (1B): not used to convey an address in LAPB (point-to-point). 0x01: command from DTE to DCE, or response from DCE 0x03: command from DCE to DTE, or response from DTE Control field Information FCS Flag 35 End-to-end arguments The principal argument: Certain functions can only be implemented completely and correctly at the transfer end-point Implementations elsewhere may be justified as performance enhancements J. Saltzer, D. Reed and D. Clark: End-to-end arguments in system design ACM Trans. Computer Systems, 2(4):277-88, Nov

6 File transfer example In a file transfer, integrity and secrecy can be compromised at various points on the end-to-end path Source computer Application Transport ❶ Router ❷ Local (re)transmission may aid performance Destination computer Application Transport ❸ Performance implications Error checks within the network could aid or impede performance. Local checks: Save bandwidth by confining the propagation of: errored information retransmitted information Allow matching of error control to link characteristics Can be integrated with other bit manipulations, e.g. Ethernet CRC Require processing within the network Added complexity Store-and-forward delay (only significant if no output queuing) May be inappropriate for apps that tolerate errors, e.g. voice Ethernet (802.3) verifies integrity, but doesn t recover Wi-Fi (802.11) verifies integrity and recovers contrary to e2e, but powerful marketing as wire-equivalent Additional end-to-end arguments For correctness; extreme form: fate sharing Appropriate service, e.g. ARQ may delay voice Simpler network (more general?) Faster operation & reduced development time More transparent, easier to understand Functions that aren t in the network won t interfere with new apps User-pays (but aggregate may pay more) Ease of deployment, e.g web-browser plug-ins Decentralist politics Frame Relay Another CCITT standard, driven by ANSI (representing US) Initially (1988) to convey signalling information over ISDN D channel (called LAPD) Later, intended as a public service for LAN interconnection e.g. Frame Relay interface cards on routers LAPF ( Access Protocol for Frame Relay) Designed to operate faster than X.25 by simplifying network processing: eliminate error recovery Often operates at 2Mb/s, and up to 45Mb/s Flag (standard 0x7E) for framing 10b Data Connection Identifier (DLCI) identifies the virtual circuit only local significance, i.e. DLCI can change as frame propagates C/R: not defined EA: Extend Address: set to 1 in last addressing byte (0 in others) FECN, BECN, DE: Discuss these soon Information: Up to bytes (FCS may limit length to 4096B) FCS: 16-bit CRC Flag for framing Frame relay frames FR congestion control Congestion control is important in Frame Relay because there is no reservation of resources network can readily become congested + designed for bursty LAN interconnect Two techniques: Consolidated -Layer Management (T1.618) switch sends congestion indications to a source using DLCI 1023, indicating the type of congestion and DLCIs involved Explicit Congestion Notification (ECN) 43 44

7 Explicit Congestion Notification (ECN) FR Traffic Management Forward ECN: On path from source to destination Switches that are nearing congestion set this bit. Downstream nodes (in particular, destination) learn about the congestion (provided the frame isn t lost, e.g. due to congestion); hopefully tell source Backward ECN: Switch that is, or is nearing, congestion sets this bit, assuming that dest of this frame may send frames through this switch. Signal may experience less delay reaching source faster response to congestion. Signal may propagate when frames are being lost due to congestion Source may not have intended sending anything, anyhow Implicit congestion notification: e.g. TCP source detects loss through missing ack and interprets as congestion (FR provides no SNs to indicate loss) 2001: Internet protocols adopt ECN (RFC 3168) 45 Client and provider agree (during signalling) on a leaky Bucket traffic profile: T c : Interval B c : Committed Burst Size B e : Excess Burst Size: Committed Information Rate (CIR) = B c /T c Counter C incremented by length of each frame transmitted C=C-B c every T c, saturating at 0 C<B c transmit frame with DE=0 B c C<B e transmit frame with DE=1 Discard Eligibility (DE) bit indicates that frame is in excess of Committed Information Rate, and so should be discarded first. (DE may also be set by policer.) 46